In a wave generator of a strain wave gearing, a ball bearing having flexible inner and outer rings capable of flexing in a radial direction is commonly mounted on an ellipsoidal external peripheral surface of a rigid plug. For the sake of reducing the number of components and making the wave generator smaller and more compact, the strain wave gearings being proposed have an inner ring trajectory surface formed directly in the ellipsoidal external peripheral surface of the rigid plug, skipping the inner ring. Patent Document 1 proposes a winch for civil engineering and construction machinery that incorporates the strain wave gearing of this configuration. Patent Document 2 proposes a strain wave gearing in which an inner ring trajectory surface is formed directly in the ellipsoidal external peripheral surface of the rigid plug, and an outer ring trajectory surface is formed directly in an inner peripheral surface of a flexible externally toothed gear, skipping the inner and outer rings.
The strain wave gearing disclosed in Patent Document 1 is known as a flat strain wave gearing, wherein a cylindrical flexible externally toothed gear is disposed on the inner sides of two rigid internally toothed gears having different numbers of teeth, and the flexible externally toothed gear is made to flex into an ellipsoidal shape and meshed with both rigid internally toothed gears by a wave generator disposed on the inner side of the flexible externally toothed gear.